Seeking geographical North is a stage prior to any operation concerning aiming, plotting or spotting, guidance, or navigation, in order to determine a reference heading from which aiming or navigation data is to be determined. When guidance and navigation are performed by means of a system including an inertial unit or an inertial sensor module, the system becomes operational once it has been “aligned” on this heading.
It is known to determine a heading in the North direction by using a North seeker that comprises a free gyro firmly secured to the ground in a horizontal position. The North seeker determines accurately the North direction in the absence of the gyro moving relative to the surface of the Earth. The accuracy of the determination depends mainly only on the drift of the gyro, or the stability of the drift when two measurements are performed on opposite headings. In contrast, the smallest amount of movement of the gyro, even when not perceptible for a human being, rapidly degrades this accuracy and falsifies North seeking. A North seeker must therefore be particularly well installed in order to avoid any such movements.
With equipment fitted with a complete inertial sensor module suitable for inertial navigation, it is known to determine a heading in the North direction by making use of data from the inertial sensor module in a gyro-compass mode while the equipment is in a stationary position relative to the surface of the Earth. An inertial sensor module has three accelerometer measurement axes and three rate gyro measurement axes. Aligning the equipment then consists either in bringing the inertial sensor module into a predetermined position relative to a local or inertial geographical frame of reference and then physically maintaining it in that position, or else in using calculation to estimate angular differences between the local or inertial geographical frame of reference and the inertial sensor module, and then to update them.
When using differences, the combined use of data both from accelerometers and from gyros, associated with practically no movement relative to the Earth makes it possible to perform alignment and thus to identify and maintain the heading of the core of the sensor module. Nevertheless, the accuracy of the heading as determined in this way depends on the accuracy of the accelerometers, and accelerometer price increases rapidly as a function of the required accuracy. For given accuracy, a North seeker is much less than expensive than a gyro compass.
By extension, it should be observed that the term “gyro compass” is used both to cover both the method of calculation and the technical device specially arranged for performing the calculation. The same applies to the term “North seeker”.